How often may I get a Pike Pike from underwater fishing? My fishing level is 6.
Then you have a 0.00% chance of catching a Pike Pike.
The items you get are based on the fishing level by plateaus, like the str/def boosts. I think the lowest you can get a PP at is...at least 60, probably higher. And even then, being higher level doesn't necessarily give you better chances at good items. My best pet is fishing level 111, and best I've gotten has been the Titanic Gigantic Squid.
I'm at 106 fishing skill, and have also never seen anything better than the TGS.
My pets are all aound levels 125-130, and the best thing I've gotten is a Flash of Rainbow Fountain Water -- and that was around level 90. Nothing good since then.
There is an excellent petpage
here that lists all the items, their uses, and the levels at which you can catch them.

Awww it's gonna take forever then...
my pet has fishing level of 119 and i still havent got a titanic gigantic squid yet
i guess it all depends on luck
so what level can you start getting TGS at, im on 83 now
-oh, duh, thnx
It's all on that petpage I linked above.
You reel in your line and get...
Pike Pike!
well, hey, i guess im in no rush to get that TGS anymore.